Flexography is a method of printing that is commonly used for high-volume printing runs. It is usually employed for printing on a variety of substances particularly those that are soft and easily deformed, such as paper, paperboard stock, corrugated board, polymeric films, fabrics, plastic films, metal foils, and laminates. Course surfaces and stretchable polymeric films can be economically printed by the means of flexography.
Flexographic printing plates are sometimes known as “relief printing plates” and are provided with raised relief images onto which ink is applied for application to the printing substance. The raised relief images are inked in contrast to the relief “floor” that remains free of ink in the desired printing situations. Such printing plates are generally supplied to the user as a multi-layered article having one or more imageable layers coated on a backing or substrate. Flexographic printing can also be carried out using a flexographic printing cylinder or seamless sleeve having the desired raised relief image. These flexographic printing cylinder or sleeve precursors can be “imaged in-the-round” (ITR), either by using a standard photomask or a “laser ablation mask” (LAM) imaging on a photosensitive plate formulation, or by “direct laser engraving” (DLE) of a plate precursor that is not necessarily photosensitive.
U.S. Pat. No. 5,719,009 (Fan) describes elements having an ablatable layer disposed over photosensitive layer(s) so that after image ablation, UV exposure of the underlying layer hardens it while non-exposed layer(s) and the ablatable mask layer are subsequently washed away.
DuPont's Cyrel® FAST™ thermal mass transfer plates are commercially available photosensitive resin plate precursors that comprise an integrated ablatable mask element and require minimal chemical processing, but they do require thermal wicking or wiping to remove the non-exposed areas. These also require extensive disposal of polymeric waste and some drying of the processed (developed) plates.
There remains a need for a totally processless method of producing flexographic printing plates with high throughput efficiency. A method for forming a relief pattern on a printing element by directly engraving (DE) with a laser is already used to produce relief plates and stamps. However, the requirement of relief depths in excess of 500 μm challenges the speed at which these flexographic printing plate precursors can be imaged. In contrast to the laser ablation of the CTP mask layers atop the photosensitive resin, which only requires low energy lasers and low fluence, the DE of laser ablatable flexographic printing plates requires higher energy lasers and higher fluence. In addition, the laser ablatable, relief-forming layer becomes the printing surface and must have the appropriate physical and chemical properties needed for good printing. The laser engravable black mask layer is washed away during the development and is not used during the printing.
Flexographic printing plate precursors used for infrared radiation (IR) laser ablation engraving must comprise an elastomeric or polymeric composition that includes one or more infrared radiation absorbing compounds. When the term “imaging” is used in connection with “laser engraving”, it refers to ablation of the background areas while leaving intact the areas of the element that will be inked and printed in a flexographic printing station or press.
Commercial laser engraving is typically carried out using carbon dioxide lasers. While they are generally slow and expensive to use and have poor beam resolution, they are used because of the attractions of direct thermal imaging. Infrared (IR) fiber lasers are also used. These lasers provide better beam resolution, but are very expensive. IR laser engravable flexographic printing plate blanks having unique engravable compositions are described in WO 2005/084959 (Figov).
Direct laser engraving is described, for example, in U.S. Pat. Nos. 5,798,202 and 5,804,353 (both Cushner et al.) in which various means are used to reinforce the elastomeric layers. The reinforcement can be done by addition of particulates, by photochemical reinforcement, or by thermochemical hardening. U.S. Pat. No. 5,804,353 describes a multilayer flexographic printing plate wherein the composition of the top layer is different from the composition of the intermediate layer. Carbon black can be used as a reinforcing agent and can be present in both layers.
Flexographic printing plate precursors for near-IR laser ablation engraving generally comprise an elastomeric or polymeric system that is made thermosetting by a polymerization reaction and includes fillers and infrared absorbing compounds. During recent years, infrared laser diodes have been used for ablation of thin layers (U.S. Pat. No. 5,339,737 of Lewis et al.) for use in offset lithographic printing. These lasers are becoming increasingly inexpensive and more powerful and consequently are becoming more useful for laser ablation of thick layers such as are found in flexographic printing precursors. Such lasers require the presence of radiation absorbing dyes or pigments in the flexographic printing precursors as they generally operate around wavelengths of 800 nm to 1200 nm. They have the potential to enable faster imaging, higher print quality, and more reliable engraving than obtained with carbon dioxide lasers. In addition, it is advantageous to optimize imaging speed by formulating printing plates with higher sensitivity. This will give higher productivity in printing plate production with the potential of greater profits for the printing houses or trade shops where the printing plates may be produced. Imaging systems can be made by using arrays of laser diodes. Throughput also depends on the number of laser diodes being used and there is a balance between the cost of imaging heads that depends on the number of diodes and their combined output power. The requirement for high print quality has increased considerably in recent years as flexographic printing penetrates markets previously dominated by high quality offset lithography. Laser engraving using infrared diodes instead of carbon dioxide provides an opportunity for higher quality because the wavelength of the diode radiation at 800-1000 nm is so much smaller than that of carbon dioxide of 10.7 μm.
As mentioned above, the chosen commercial means of imaging by laser engraving has for some years been with carbon dioxide lasers. These are capable of ablating layers to produce suitable relief depths for flexographic printing. Such depths may be anywhere in the range from 200 μm to 5 mm. As carbon dioxide lasers operate at a wavelength of 10.7 μm, there is no need to incorporate infrared absorbing dyes or pigments into the printing precursors because the polymers themselves absorb at this wavelength for ablation.
Although patents concerned with formulating laser-engraved printing precursors may mention laser diode engraving, they have been primarily aimed at carbon dioxide laser imaging and thus include formulations lacking infrared absorbing materials as described in U.S. Pat. No. 5,259,311 (MacCaughey). Formulations designed for ablation by carbon dioxide lasers cannot be easily modified for laser diode ablation by simply adding a suitable infrared radiation absorbing material. For instance, infrared dyes may react with the chemistry used to vulcanize the last-ablatable layer, or carbon black may block UV radiation used for curing the flexographic precursor composition.
One approach to formulation of laser-engravable flexographic printing precursors is to produce thermoplastic formulations that have not been crosslinked to form thermoset materials. These have been found to be of limited suitability for laser engraving because ablation of thermoplastic materials results in melted portions around the ablated areas and sometimes re-deposition of ablated material onto the ablated areas. This is because it is inevitable that during imaging there is heat flowing to non-imaged areas that is insufficient for ablation but sufficient for melting, as described in U.S. Patent Application Publication 2004/0231540 (Hiller et al.).
A number of elastomeric systems have been considered for construction of laser-engravable flexographic printing precursors. The earliest formulations included natural rubbers (as reported in U.S. Pat. No. 6,223,655 by Shanbaum et al. using a mixture of epoxidized natural rubber and natural rubber). Also, engraving of a rubber is described in “Laser Material Processing of Polymers” by S. E. Nielsen in Polymer Testing 3 (1983) 303-310.
U.S. Pat. No. 4,934,267 (Hashimito) describes the use of natural rubber or synthetic rubber or mixtures of both and specifically mentions acrylonitrile-butadiene, styrene-butadiene and chloroprene with a textile support. “Laser Engraving of Rubbers—The influence of Fillers” by W. Kern et al., October 1997, pp. 710-715 (Rohstoffe Und Anwendendunghen) described the use of natural rubber, nitrile rubber (NBR), ethylene-propylene-diene terpolymer (EPDM), and styrene-butadiene copolymer (SBR). The article entitled “Laser Engraving of Rubbers—The Use of Microporous Materials” by Kern et al., 1998 described the use of natural rubber compounds and EPDM.
EP1,228,864B1 (Houstra) describes liquid photopolymer mixtures designed for analogue UV imaging, cured with UV, and then the resulting plates are laser engraved using carbon dioxide. Such printing plate precursors do not contain infrared absorbing dyes or pigments and therefore are unsuitable for use with IR emitting laser diode systems. U.S. Pat. No. 5,798,202 (noted above) describes reinforced block copolymers incorporating carbon black that is UV cured and is still thermoplastic. As pointed out in U.S. Pat. No. 6,935,236 (Hiller et al.), such curing would be defective due to the high absorption of UV as it traverses through the thick precursor layer. The block copolymers described in Cushner et al. are the basis of most commercial UV-imageable flexographic printing precursors. Although many polymers are suggested for this use in the prior art, only polymers that are extremely flexible such as elastomers have been used commercially. This is because flexographic layers that are millimeters thick need to be bent around a printing cylinder and secured with temporary bonding tape that must both be removable after printing and secure the printing plate during printing.
U.S. Pat. No. 6,776,095 (Telser et al.) lists a number of elastomers including EPDM and U.S. Pat. No. 6,913,869 (Leinenbach et al.) describes the use of EPDM rubber for the production of flexographic printing plates having a flexible metal support. U.S. Pat. No. 7,223,524 (Hiller et al.) describes the use of natural rubber with highly conductive carbon blacks with specific properties of structure and surface area. U.S. Pat. No. 7,290,487 (Hiller et al.) lists suitable hydrophobic elastomers with inert plasticizers. U.S. Patent Application Publication 2002/0018958 (Nishioki et al.) describes a peelable layer and the use of rubbers such as EPDM and NBR together with inert plasticizers such as mineral oils. The use of inert plasticizers or mineral oils can present a problem as they leach out either during precursor grinding (during manufacture) or during storage, or under the pressure and contact with ink during printing.
An increased need for higher quality flexographic printing precursors for laser engraving has highlighted the need to increase the desire to solve performance problems that may have been of less importance when quality demands were less. What is especially difficult is to simultaneously improve the flexographic printing precursor in all directions.
For example, the rate of imaging is now an important consideration in laser engraving of flexographic printing precursors. Throughput by engraving depends upon printing plate width because it is imaged point by point. Conventional printing plates made by UV exposure followed by multiprocessing wash-out and drying is time consuming but is independent of printing plate size, and for the production of multiple printing plate, it can be relatively fast because many printing plates can be passing through the multiple stages at the same time. Throughput for flexographic engraving is somewhat determined by the equipment that is used but if this is the means for improving imaging speed, then cost becomes the main factor. Improved imaging speed is related to equipment cost. There is a limit to what the market will bear in equipment cost in order to have faster imaging. Therefore, much work has been done to try to improve the sensitivity of the flexographic printing plate by various means. For instance, U.S. Pat. No. 6,159,659 (Gelbart) describes the use of a foam layer for laser engraving so that there is less material to ablate. U.S. Pat. No. 6,806,018 (Kanga) uses expandable microspheres to increase sensitivity.
U.S. Patent Application Publication 2009/0214983 (Figov et al.) describes the use of additives that thermally degrade to produce gaseous products. U.S. Patent Application Publication 2008/0194762 (Sugasaki) suggests that good imaging sensitivity can be achieved using a polymer with a nitrogen atom-containing hetero ring. U.S. Patent Application Publication 2008/0258344 (Regan et al.) describes laser-ablatable flexographic printing precursors that can be degraded to simple molecules that are easily removed.
There continues to be a need to provide improved flexographic printing precursors that are easily manufactured without the use of process oils that have improved sensitivity (imaging speed) and provide improved print quality and run length.